As a non-native English speaker, ages ago I moved to The Netherlands (were they also use “ja” for “yes”) and once I learned Dutch and got used to speak it as much or more than English, I noticed a definite tendency on my English for my “yes” to come out quite “ja”-like (sorta like an “yeah” with a pretty much silent “e”), though granted not as strong as that guy.

Maybe this is some kind of broader linguistic tendency (non-native English speakers used to a “yes” in a different language that’s pretty close to one of the English words for “yes” - in this case “yeah” - just doing the lazy thing of using the other language word or a softened version of it because English-speakers get it) rather than a German-specific thing.

I would be curious to hear from Dutch people and people from Scandinavia (if I’m not mistaken most if not all those languages use a “ja” for “yes”) if they tend to do that or not.